Fragments of colonies of branched Acropora species depend for survival on their stability, which is strongly enhanced through cementation of the fragments to hard substrata. On Atlantic reefs a major part of the hard substrata available after fragmentation by disturbances are fragments of conspecific or congeneric colonies. The authors studied the reactions within and between colonies of Acropora palmata (Lamarck) and between A. palmata and A. cervicornis (Lamarck) in situ. Isografts showed confluent tissues and wholly fused skeletons within 10 wk. Allo- and xenografts displayed simple overgrowth of fragments and branches. Although the tissues remained discontinuous and fusion of skeletons was not complete, there was a significant degree of cementation. No antagonistic reactions were found. The absence of strong antagonistic reactions, mechanisms employed in spatial competition, has adaptive value for branched corals in this shallow water habitat. The results indicate that colonies in A. palmata populations are at least partly derived from asexual propagules.